The present application is directed to a set screw for use in applications where it is important for the set screw to have a clean and low profile after insertion, but for the set screw to grip the object against which the set screw sets and be removable, especially set screws for use in conjunction with medical implants.
Over recent years, medical implants for support of and correction of defects in various portions of the body, especially the spine, have improved dramatically. It is desirable to continue to improve these implants, so that the implants provide greater strength and durability and so that the implants are effective for the life of the patient. Furthermore, it is very desirable to provide such an implant which has low profile and interferes as little as possible with the living tissue surrounding the implant. In the past the set screw that secures together various parts of the implant, especially spinal implants, has often been a limiting factor with respect to such implants, the present invention is directed to improving such set screws.
Set screw manufacturers, especially for devices such as medical implants, are continually faced with the challenge of providing a set screw with the often conflicting goals of being small, lightweight, low profile and unobtrusive, while also being relatively quite strong and durable. Furthermore, it is desirable that the set screw be able to lock together two elements which very commonly include a rod positioned in a second medical implant. If the elements, such as rods, move axially or rotationally with respect to the other implants, the entire implant system may fail and may cause additional injury to the person within whom the implant is installed. Set screws used in spinal implants is are mainly of a first type that are screwed into a closed and threaded bore or a second type that are screwed between threaded surfaces of a pair of upstanding spaced arms in an implant which are often referred to as a plug.
Set screws used in medical implants are typically seven to ten millimeters in diameter. However, it is desirable to have yet smaller screws. One method of reducing the size of the screw without decreasing the strength is to decrease the outer diameter of the thread with or without changing the inner diameter of the thread by, in general, having the thread depth decrease. The overall strength of the implant is generally related to the inner diameter of the thread. Consequently, comparatively increasing the inner diameter or maintaining the inner diameter at the same diameter while decreasing the outer diameter can be effectively used to strengthen the set screw for a specific outer diameter.
When the inner diameter of the thread is increased or the outer diameter of the thread is decreased and the inner diameter remains the same, the pitch of the thread, which is the width between two peaks of the thread and which is sometimes measured in the number of threads per inch, is increased. That is, the thread becomes finer and, the depth of the thread becomes less and the distance between the peaks of adjacent turns of the thread becomes decreased. While increasing the thread inner diameter does increase the strength of the set screw, there is a point of diminishing returns whereat the thread depth is too little to grip between opposed threads and there is slippage. It has been found through experimentation that the thread depth which gives the greatest strength and yet does not slip may vary somewhat with size. Thus it has been found that in a 6 mm. diameter set screw, a pitch ratio of 0.4 may allow slippage whereas a pitch ratio of 0.5 to 0.55 provides substantial strength and resists slippage.
Not only does the finer thread aid in increasing the strength of the set screw, it is also better facilitates threads of opposed threaded elements aligning with one another. In particular, as mentioned above, some of the set screws in accordance with the present invention are used with implants having spaced arms having threaded interior surfaces. In such implants the set screw engages two discontinuous threaded surfaces. This can lead to inadvertent cross-threading where one turn of the thread of the set screw is lined up with two different turns of the thread on the facing surfaces of the arms. The finer thread, as discussed above, reduces the likelihood that a surgeon will inadvertently initiate cross-threading when the surgeon begins to install the set screw into a threaded receptacle for the screw.
As mentioned above, it is also necessary for the set screw to tightly grip whatever implant element it is urged against so as to lock that element relative to a second element within which the set screw is threaded. Such locking is partially provided by friction. However, positive penetration of the set screw into the element to be set assists in the locking and provides for a more secure lock. A smooth circular surface on the underside of the set screw does not provide digging into or abrasion of the element to be locked and such smooth bottom set screws must rely solely upon the friction generated between a fairly smooth surface and the other element such as a rod for secure locking. In order to overcome this problem, prior art has utilized various structures on the end of the set screw such as points, knurling and cutting rings.
Applicant has found another mechanism for increasing the friction between the set screw and the element to be locked which is to include slots that extend from the sides of the set screw inwardly and open onto the lower face of the set screw. At the intersections of the slots with the bottom face of the set screw are edges which tend to dig or cut into the element to be locked under relatively high torque. Consequently, the set screws incorporating such slots resist greater forces trying to turn a set element relative to the implant in which the set screw is threaded as compared to set screws having solely smooth circular lower surfaces.
A comparatively strong, but lightweight and low profile set screw that may be used in many applications including in a threaded surrounding bore or between a pair of threaded spaced arms of a first element of an implant to lock or secure a second element of the implant to the first element thereof. The set screw comprises a generally cylindrical body having an outer surface that is threaded, an upper surface and a lower surface. The set screw also includes a structure for driving and torquing the set screw which may include a retained head, a breakaway head, a penetrating bore with polyhedral sides, or upward opening slots or bores within the set screw that each may be utilized in conjunction with a tool for turning and torquing the set screw.
The set screw includes at least one and preferably a pair of side slots. The side slots extend inwardly from the outer threaded surface of the set screw. Furthermore, each side slot intersects with a lower surface of the set screw such that there is a pattern of edges of the slots along the set screw lower surface. Under torque the edges of the slots that are at the lower surface of the set screw preferably dig or cut into and grip the second element to be fixed by the set screw.
In one embodiment of the invention the side slots extend parallel to the axis along the entire length of the set screw. After the set screw is installed, for example, by use of a break-away head which shears away from the remaining body of the set screw at a preselected torque thereby exposing the side slots or by use of a tool that is received in the slots, the side slots may be accessed by a tool having a similar cross-section to the slots opening upward for purposes of removal.
The side slots may have a variety of cross-sections including rectangular, arcuate or pie-shaped.
In yet another embodiment of the invention, the side slots may be discontinuous in that a lower side slot intersects with the lower surface of the set screw and an upper side slot intersects with the upper surface of the set screw, but the lower and upper slots are discontinuous or non-intersecting with one another. In this manner the lower side slot provides a gripping of the element to be set and the upper side slot allows for removal.
The pitch of the thread is comparatively preferably increased in order to reduce cross threading upon installation. Further, the depth of the thread is preferably decreased to improve the strength of the thread, but is maintained sufficient to prevent slippage between the two threaded surfaces. For a set screw having a 5 to 5.5 mm. diameter a pitch ratio of 0.4 to 0.5 is preferred.
Therefore, the objects of the present invention are: to provide a set screw especially for use in conjunction with medical implants and the like wherein the set screw is comparatively strong and yet has a low profile after installation, allows for installation to a preselected torque and is removable from the implant after installation; to provide such a set screw having at least one side slot that extends radially inward from an outer threaded surface thereof and which intersects with a lower surface of the set screw that engages a first element to be set by the set screw such that edges of the slot grip the first element upon torquing of the set screw; to provide such a set screw where there are a pair of opposed side slots; to provide such a set screw having side slots that extend parallel to the axis of the set screw the entire length of the set screw, or alternatively, having separate upper and lower portions with one portion intersecting with the lower surface of the set screw and a second portion intersecting with the upper surface of the set screw such that the portion that intersects with the lower surface aids in gripping and the portion that intersects with the upper surface may be used in installation and/or removal of the set screw by use of a tool; to provide such a set screw which has an outer generally cylindrical surface except where intersected by the side slots and that has a thread with multiple turns of the threads laid thereon in a helical pattern except in the region of said side slots; to provide such a set screw wherein thread depth is comparatively decreased and pitch is increased to produce a finer thread to increase strength of the set screw without increasing size and to reduce the likelihood of cross-threading during installation; to provide such a set screw that may be used equally in conjunction with closed threaded bores or open heads wherein the set screw is utilized as a plug between a pair of inwardly threaded and spaced arms; to provide such a set screw that may be utilized with a breakaway head that breaks from a base of the set screw at a preselected torque, or alternatively, may be used with a retained head that allows torquing to a preselected torque; to provide such a set screw that also alternatively allows installation by use of a tool, either in side slots intersecting with the upper surface of the set screw or with alternative bores in the upper surface of the set screw; and to provide such a set screw which is comparatively easy to manufacture, inexpensive to produce and easily utilized by surgeons during spinal surgery and especially well adapted for the intended uses thereof.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.